united states customary units are a system of measurements commonly used in the united states. the united states customary system (uscs or usc) developed from english units which were in use in the british empire before the u.s. became an independent country. however, the united kingdom's system of measures was overhauled in 1824 to create the imperial system, changing the definitions of some units. therefore, while many u.s. units are essentially similar to their imperial counterparts, there are significant differences between the systems.
the majority of u.s. customary units were redefined in terms of the meter and kilogram with the mendenhall order of 1893 and, in practice, for many years before. these definitions were refined by the international yard and pound agreement of 1959.
americans primarily use customary units in commercial activities, as well as for personal and social use. in science, medicine, many sectors of industry, and some of government and military, metric units are used. the international system of units (si), the modern form of the metric system, is preferred for many uses by the u.s. national institute of standards and technology (nist). for newer units of measure where there is no traditional customary unit, international units are used, sometimes mixed with customary units, such as electrical resistance of wire expressed in ohms (si) per thousand feet.